Water-soluble cellulose ethers, e.g. methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, hydroxyethylcellulose, carboxymethylcellulose, have found widespread use as adhesives, thickeners or protective colloids. However, introduction of the cellulose ether which is normally in dry powder form into water or aqueous systems is frequently associated with problems, since these substances tend to gel on the surface when added to water, and form lumps. This results in undesirably long dissolution times.
It has been known for a long time that the dissolution behaviour of cellulose ethers can be influenced by means of an after- or post-treatment.
U.S. Pat. No. 2,684,914, for example, describes a method of increasing the viscosity of carboxymethylcellulose (CMC) by use of formaldehyde and sodium hydroxide. The aim of this method is to permanently alter the viscosity of CMC compared with the untreated CMC.
Various methods of influencing the dissolution behaviour by use of surface-active substances are also known, such as described in U.S. Pat. No. 2,647,064 or U.S. Pat. No. 2,720,464.
U.S. Pat. No. 2,879,268 describes a process for treating cellulose derivatives in solid form with formaldehyde or glyoxal at a low pH in order to minimize the tendency of the cellulose derivatives to agglomerate during dissolution.
DE-A-1 051 836 describes a process for treating water-soluble cellulose ethers which is characterized in that the cellulose ether, in finely divided powder form, is treated with polyfunctional compounds.
U.S. Pat. No. 3,072,635 describes a process for preparing water-dispersible cellulose derivatives by treatment of such cellulose derivatives with glyoxal.
DE-A-1 239 672 describes a process for preparing pulverulent methylcellulose which is soluble in water without lump formation, in which the methylcellulose is kneaded with a dialdehyde and water at a pH of from 3 to 7 and is subsequently dried and comminuted.
U.S. Pat. No. 3,489,719 describes a process for the surface treatment of dry water-soluble cellulose derivatives using a dialdehyde, a fatty acid ether and an acid catalyst.
WO 99/18132 describes a process for preparing polysaccharide derivatives which display delayed dissolution in water by means of glyoxal, which is characterized in that part of the energy required is introduced in the form of electromagnetic radiation.
Ullmann's Encyclopedia of Industrial Chemistry, Volume A5, 1986, pp. 472–473, describes methods of delaying the dissolution of methylcellulose and mixed methylcellulose ethers. Aqueous glyoxal having a pH of from 4 to 5 is used to delay dissolution.
The majority of these methods employ acid catalysts and often require multistage drying or additional treatment steps after drying and comminution. They frequently lead to an undesirable decrease in viscosity with increasing storage time. In addition, they are typically inapplicable to both ionic and nonionic cellulose ethers, so that various mixtures of chemicals have to be kept available to delay the dissolution of ionic and nonionic cellulose ethers.
There is therefore still a need to alleviate all these disadvantages.